Production of refractory oxide ware



Patented Wiley 25, 192%..

JOHN WESLEY MARDEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOB T0 WESTING- HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

PRODUCTION OF REFRACTORY OXIDE HARE.

No Drawing.

The present invention relates to the produclion of refractory ware and more particularly relates to a method of producing crucibles and other articles of thoria, zirconia and the like, and the product thereof.

An object of my invention is the provision of a method for econon'iically and expeditiously producing crucibles and other articles, of refractory material such as thoria or zirconia. a

A further object of my invention is the provision of a compound containing a refractory material such as thoria and the like which may be'conveniently employed in casting crucibles, pots, tubes for tungstenwound furnaces and the like.

A further object of my invention is the provision of a casting process for the production of thoria crucibles.

A further object of my invention is the production of a cast crucible composed primarily of thoria.

()ther objects will be apparent from a reading of the following description.

In practicing, many metallurgical processes, it is desirable to prevent the contamination of the material undergoing treat ment. This is particularly true when investigations are being made of the metals which require high-heat treatments. It is known, for instance, that magnesia, lime and even zirconia cannot be used in contact with tungsten 0r molybdenum on account of the interaction between the metal and the oxide. In the case of a lime crucible, for example, tungsten interacts with this oxide to yield volatile products which are probablv calcium metal and tungstic oxide. For this reason and many others, attempts have been made to produce crucibles and other articles, used in connection with metallurgical investigations. from the refractory oxides, especially thorium oxide and zirconium oxide. Inasmuch as thorium oxide. when highly heated is one of the most rc- Fractory of substances, above that of platinum, and also because its chemical resistivity is very great, it will be appreciated that a vessel, such as a crucible or a tube for an electric 'l'urnace, composed of practically the pure oxide will lind considerable usage in fusing metals havin" relatively high melting points, such as t iorium and zirconium. and also in practicing other metallurgical processes, as, for instance, the determina- Serial No. 542.134.

tion of silica in the chemical laboratory. Another important factor which is l'avorahlc to the employment of thoria and zirconia in crucible manufacture is that these compounds have low coellicients ol' expansions. It is also believed that a substantially pure thoria crucible would be of considei able advantage in treating certain ores, because direct treatment with hydrofluoric acid could be resorted to.

As already stated, many attempts have been made to produce crucibles from refractory materials, such as magnesia, thoria, zirconia, etc, but, so far as 1 am aware, no one has been able to manufacture these articlcs on a commercial scale because of the difliculties encountered in handling these nonplastic materials. In certain of the processes practiced, recourse has been had to the employment of a binder such as starch, gclatine, gum and the like. The principal objection to the use of these binders is that impurities are thereby introduced into the linishcd article which, obviously. may seriously intcrl'ere with the successful practicing ot' a metallurgical process. An illustration of this has already been given when tungsten is undergoing treatment in a lime. crucible. Many other instances might be cited, especially where the crucibles are contaminated with graphite or carbon.

So far as I am aware, no successful cast-- ing' process has been devised for the production of refractory crucibles free from carbonaceous and siliceous substances. The advantages derived from having a comparatively pure refractory crucible are obvious from the foregoing: description.

Briefly stated, the process of casting, as practiced by me in producing crucibles of substantially pure refractory substances, consists in forming a suspension of suitable consistency for casting. This suspensioi1,wliicli is usually termed the slip in the pottery industry, is prepared bv thoroughly intermingling, by means o l a ball mill, the refractory substance of Which thec'rucible is principally composed, a small. quantity of another refractory material, to serve as a tiller, a binder salt, preferably a double fluoride salt, which functions similarly to a colloid, and a small quantity of a suitable acid which will cause any colloid present to coagulate.

of the refractory substance which has been previously pulverized until quite fine. This mixture is diluted a little with water to give a suitable consistency for casting. This suspension, which is termed the slip is poured into a plaster of Paris mold having the proper degree of moisture. The mold is kept full for a few seconds or minutes, depending upon the dryness of the form, the proportion of water in the slip and the thickness of walls desired on the crucible. lVheu the desired thickness is obtained, the excess slip" is poured out. The form and cast crucible are allowed to dry and harden. In drying. the crucible shrinks away from the walls of the casting form and may then be turned out into the hand. After being care fully dried. the crucible is tired atabout litlti (1. at which time most of the salts are volatilized and the crucible takes on a permanent set.

(.are should be exercised in having the proper degree of moisture in the "asting forms. as there is a tendency for the cast ing to stick or crack it the form is too dry, whereas. if it is too wet. good results are not obtainable.

Another important consideration is the drying of the casting. for. it the drying, step is carried on too rapidly, the casting will crack. Generally speaking. the drying depends on the size. of the article. the larger the vessel. the longer the time required for drying. Furthermore. the drying temperatur should be gradually raised until "the maximum temperature has been reached at which temperature the drying operation should be continued for a short interval.

The proportions of binder salt and the fineness of sub-division may be varied within quite liberal limits, and therefore, no directions can be set down with much exactnose.

A specific example may be helpful in order to understand the process practiced by me. It: is to be understood that. in practicin this process, variations may be made in the quantities of materials employed. the nature of such materials, and the periods of time required for certain of the operations.

In casting thoria crucibles. l first prepare a mixture of approximately nine parts of horia which has been previously ignited at i700 to 1900 about one part of Zirconia. about one part oi potassium-thorium fluoride. potassium-zirc mium fluoride. crvolitc. and the like. and a onohalt' part of pl'iosphoric or other suitable acid. 'lhe mixture is ballmilled wet and. after it has been ground very finely in this way. it is mixed with an equal part of the ignited thoria which has been pulverized in the mortar to pass a 200 mesh sieve. This mixture is diluted with water to give a suspension of suitable consistency for casting.

The mold or casting form into which the slip is poured to form the casting, is prepared, preferably out of plaster of Paris. A solid block of plaster of Paris is turned. at one end on a lathe to obtain the desired shape. A. soft-soap solution is deposited over the block and the protuberance formed by turning, and the Solution permitted to dry. The block is placed on a piece 0t board, protuberance up, surrounded with a piece of iron or card board. A mixture 025 four parts of plaster oi Paris and three parts of water is poured into the. cup thus lOl'lDGtl and allowel to dry. \Vhen hard, the two pieces of plaster of Paris are separated. The one having! the depression is retained and serves as the casting form or mold. When this form is nearly dry, it is ready for casting crucibles.

The slip is poured into the mold and the latter kcpt full until the proper thickness of wallsdesired on the crucible has been obtained. The slip hardens adjacent the mold, the central portion retainin; its plastic condition. 'hen the desired thickness has been obtained the excess slip is poured out and the excess solidified thoria cut away from the top of the casting form.

The form and contents are allowed to dry and harden. 'lhe crucible shrinks away from the walls of the casting form and atter sutlicient time may be turned out into the hand and allowed to further dry in the air. it is important. in drying. to avoid cra king the crucible and, therefore. I preler to u'occcd. as follows in performing this step. Alter removal from the mold I place the crucible in a drying box having four compartments. progressively heated from the bottom to the top. the compartment having the highest temperature being located at the bottom of the box. The crucil-ile is tirst placed in the coinpartnrent having the lowest temperature. i. e... in this case, at the top, and allowed to remain for about twelve hours. ln order to avoid too rapid drying in this connuirtment. l moisten the air by plueinsr therein a receptacle containing water. 'lhe crucible is then placed in thenetit compartment tor a short time. say about two hours. and so on into the remaining compartments until practically all moisture has been eypclled. The temperature of this oven should range from room temperature to about (30 C.

The thoroughly dried crucible is then placed. in a drying oven and heated to about 120 C. to expel the remaining moisture. This heat treatment is continued for about two hours. or until, practically all moisture has been expelled.

'lhe crucibles are next. tired by being" placed in a tube furnace. The temperature of the furnace is gradually increased over aperiod of about two hours until a tempertit? ature 0t 1200 C. has been obtained at which temperature the tiring is permitted to con tinue for about one hour. It is preferable to frequently turn the crucibles during tiring in order to distribute the heat uniformly and also keep them from sagging. Inorder to obtain a more unilorm heat distribution. a rotary kiln may be employed. During the. firing proi-ess. most of the salts volatilize, the linal ('UIHPUSltlOl'l ot' the crucible being practically pure thoria with a slight amount of zireonia. 'lhe crucible, upon removal from the. kiln, should be permitted to cool.

It is immaterial whether such cooling be rapid or slow, as the low eoetlicient of expansion ot' thoria makes the crucible less susceptible to temperature changes.

'hat is claimed is:

1. An article of manufacture composed of thoria and resulting from shaping and firing a mixture of thoria, cryolite, phos phoric acid and water.

2. An article of manufacture composed of thoria in a dense, coherent condition free from carbon. and resulting from shaping and tiring a mixture of tlioria, a small quantity of zirconia. potassium-thorium fluoride, phosphoric acid and water.

3. An article of manufacture com iosed of dense. coherent thoria free from car on and resulting from shaping and firing a mixture of about thoria, about 10% zirconia, about 10% potassiuni-thorium fluoride, about 5% phosphoric acid and sutlicient water to form a slip.

4-. A cast article eom iosed principally of thoria.

A :ast crucible composed principally of thoria.

6. A cast tboria crucible.

7. A silica-free thoria crucible.

8. A carbon-free thoria crucible.

9. A silica-andcarbon-tree thoria crucible.

10. A slip for casting articles from refractory oxides comprising a finely divided refractory oxide. 21 filler, a double salt, an

acid. and a sufiicient quantity of water orother liquid to produce a suitable consistency for casting.

ll. A slip for casting articles from refractory oxides comprising: about 80% of alinely divided refractory oxide, about 10% of a tiller, a double salt. a slight amount of an acid, and sullicient water to furnish proper consistency.

12. The method of casting thoria crucibles which consists in ball-milling wet a mixture of about nine parts of thoria, about one P'ill'i of zirconia. about one. part of potassium thorimn fluoride. and about one-half part of phosphoric ac'd, then mixing therewith an equal part of ignited thoria. subsequently forming a slip of suitable consistency for casting, by adding water, casting said slip into a. mold, removing the excess slip, drying the cast crucibles and tiring.

13. The method of casting thoria crucibles and other articles which consists in forming a slip by adding a salt, an acid, and water to finely divided thoria, depositing said slip into a mold, removing the excess slip, drying the cast articles and tiring.

ll. The. steps in the process of casting crucibles of refractory oxides which consist in ball-milling, wet a mixture of about nine parts oi thoria, about one part of zireonia, about one part of a. double salt. and about one-half parts oi phosphoric acid, then mixing therewith an equal part of ignited thoria in a finely divided state and adding sufiicient water to give the desired consistency.

15. The method of casting articles from. refractory substances which comprises intimately mixing a substance which is to form the ultimate article, a suspending medium substantiall insoluble, and water, to form a slip, casting said article from said slip, drying the cast article and firing.

16. The method of casting articles which comprises formin a slip by mixing the substance from which the article is to be formed. water, a suspending medium substantially insoluble in water and a chemical to prevent the formation of colloids, deposit ing said slip into a mold, removing, the excess slip, drying the cast article and tiring.

17. The step in the process of preparing a paste or a slip which comprises the addition to said slip of a noncolloidal suspending" medium.

18. The step in the process of preparing a paste or slip for refractory ware which comprises the addition of a suspending medium which is practically insoluble in water and which does not depend upon the action of colloids in performing its function.

19. The step in the preparation of refractories for formation into shaped articles which comprises the addition to the slip -from which said articles are to be made of a substantially insoluble non-colloidal suspending compound and an electrolyte.

20. lhe step in the process in the formation of a plastic mass from which articles are to be formed which comprises the addition to said mass while being prepared of an electrolyte to prevent the formation of colloids.

21. A suspending medium for refractory material consisting of a double fluoride salt.

22. A suspending medium for refractory material consisting of cryolite.

23. A suspending medium for a refractory compound from which articles are to be formed comprising a substantially insoluble non-colloidal substance.

24. A suspending medium for refractory substance from which articles are to be formed comprising a substantially insoluble non-colloidal salt.

25. A refractory support for the heat treatn'ieut of materials consisting of a nontragile. coherent body composed of more than 80% of thorium oxide and free from carbonaceous and siliceous materials.

26. A self supporting crucible composed of more than 80% of thorium oxide.

27. An enclosure for the heat treatment of materials consisting of a non-fragile, coherent. body composed of more than 80% thorium oxide.

28.!1 refractory support for the heat treatment of materials at high ten'zperaturcs composed of more than 807 of thorium oxide and free from ingredients capable of contaminating the materials to he treated.

29. A cast and tired article composed principally of thoria.

30. A cast and tired crucible composed principally of thoria.

In testimony whereof I have hereunto subscribed my name this seventh day of M arch 1922.

JOHN WESLEY lilARDEN. 

